Wrap dll functions more simply

* decompress.c, gnutls.c, image.c, xml.c:
If WINDOWSNT, use '#define FOO fn_FOO' to wrap dll functions,
rather than the inverse when not WINDOWSNT.  This isolates the
fn_* business into the WINDOWSNT-specific section of the code,
which makes it easier to maintain the generic code.
* decompress.c (DEF_ZLIB_FN, LOAD_ZLIB_FN):
* gnutls.c (DEF_GNUTLS_FN, LOAD_GNUTLS_FN):
* image.c (DEF_IMGLIB_FN, LOAD_IMGLIB_FN):
* xml.c (DEF_XML2_FN, LOAD_XML2_FN):
Remove.  All uses replaced by DEF_DLL_FN.
* w32.h (DEF_DLL_FN, LOAD_DLL_FN): New macros.